In many industrial processes for producing fine-particle spherical polymers or microcapsules, a droplet dispersion or cores of fine-particle, liquid or solid material surrounded by a liquid sheath is first formed. Thereafter, the droplets or the liquid sheath enclosing the particles is hardened or stabilised by adding a further liquid, e.g. a hardener or an acid or base which changes the pH value of the dispersion.
These processes are problematic because it is difficult to mix the liquid into the droplet dispersion gently enough to avoid agglomeration and coalescence of the droplets and thus to avoid disturbance of the droplet size distribution.
In the case of the widely used method of micro-encapsulation by coacervation or complex coacervation, for example, a droplet dispersion is produced in an aqueous gelatine solution or an aqueous solution of gelatine and gum arabic at a substantially neutral pH value, and the droplets are coated with a gelatine layer. Encapsulation is effected by the simultaneous addition of a copolymer and an aqueous solution of an inorganic acid, optionally followed by a reduction in the temperature of the dispersion. The capsules obtained in this way are so stable that they may be washed and optionally hardened through the addition of formalin and a simultaneous increase in the pH value. However, before acidification, the suspension of gelatine-coated droplets is very sensitive to mechanical loading, necessitating the gelatine-coated droplets to be very gently mixed with the acid solution.